The invention relates to a mold for the injection molding of objects of large dimensions with complicated surfaces from plastic material, such as plates, filter elements, bearing plates, and the like.
Molds for the manufacture of injection molded articles of large dimension are acutally made in the shape of a compact steel block, particularly as a cast or forged piece, which is worked to the required shape by machining. Openings for a cooling medium are bored into the steel block. The hollow space for injection of the plastic material is shaped in the same manner in the active part of the mold. In the technology of the structural injection molding of plastic materials, lower closing forces are used with some modifications, due to lower injection pressures that are required with respect to conventional injection molding wherein a melt of plastic material is charged into the mold. Thus it is possible to use for the mold castings of alloys oflight metals, for instance aluminum, which have a more advantageous heat conductivity than steel. A drawback of such molds, however, is the easy vulnerability of their active surfaces due to the lower hardness of the materials used.
The technology of manufacture of molds of large dimensions is very demanding. Forgings or castings require complicated machine tools for the working of all details of the construction, particularly spcial copying machines and the like. The necessity of using working tools of small dimensions substantially prolongs the time required to manufacture molds and thus also substantially increases their cost. The required shapes need both special tools and machines which are frequently not available. Thus the part of manual working on the molds increases, leading to prolonged manufacturing times, which may even amount to more than a year. Due to high costs of manufacture of the molds, it is frequently impossible to make use of some economically advantageous applications of plastic materials. Another obstacle, in the case of a rented mold manufactured at high cost, the high cost of time for the use of the mold may prevent experimentation in case of innovations in injection moldings which might be produced in the mold.
Another obstacle is the requirement of a substantial capacity of large tool plants, what can be practically solved only by costly investments.
The construction of cores of molds for the conventional injection molding of plastic materials has a number of drawbacks. There is primarily a firm connection of the cores with the mold capacity or the possibility of shifting of the cores in the mold only in the direction of the core axis. As a consequence thereof, due to the contraction forces of the cooling down melt of plastic materials, high stresses are generated in the molding, particularly in the neighborhood of the cores. In the technological process therefore, the cast is removed from the still not fully cooled mold, or the core is, in a certain moment in the course of cooling, pulled in the cirection of its axis into the internal part of the mold. These steps have the drawback that, during the following cooling down, deformations of the shape of the molding and changes of its dimensions occur, thus endangering the reproductibility of the moldings. These influences become particularly evident with moldings of large dimensions, where even a destruction of the molding may occur so that compromises have to be made in the design, or openings and recesses in the molding have to be additionally provided. This is particularly true with structural injection molding, wherein usually large dimensions and different structures of surface and internal layers of the molding are present and wherein additional working should be eliminated.
In the case of the majority of applications of injection molding of articles of large dimensions, a corrosion of the mold under stress can occur within a short time interval, the cause of which are local stresses in the mold in the neighborhood of the cores. Their prevention in molds producing moldings of large dimensions is practically impossible. The elimination of the causes of these stresses has therefore a substantial importance for the life time of the molds. The sole solution as to how to prevent the generation of stresses and of deformations of the molds is to provide for the movement of cores in the course of the shrinkage of the plastic material.